Multistage spot size converter in silicon photonics

ABSTRACT

A device is provided for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber. The device includes a waveguide comprising a waveguide taper region, which comprises a shoulder portion and a ridge portion above the shoulder portion. The ridge portion has a width that tapers to meet a width of the shoulder portion. The waveguide taper region comprises a first material. The device also has a mode converter coupled to the waveguide. The mode converter includes a plurality of stages, and each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region. The mode converter is made of a second material different from the first material.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. Nonprovisional patent application Ser. No. 16/171,132, filed on Oct. 25, 2018, entitled “MULTISTAGE SPOT SIZE CONVERTER IN SILICON PHOTONICS,” which claims priority to U.S. Provisional Patent Application No. 62/577,052, filed on Oct. 25, 2017, entitled “MULTISTAGE SPOT SIZE CONVERTER IN SILICON PHOTONICS,” the disclosures of which are hereby incorporated by reference in their entirety for all purposes.

The following are incorporated by reference for all purposes: U.S. patent application Ser. No. 14/615,942, filed on Feb. 6, 2015; U.S. patent application Ser. No. 14/722,970, filed on May 27, 2015; U.S. patent application Ser. No. 14/722,983, filed on May 27, 2015; and U.S. patent application Ser. No. 15/051,348. The above applications describe examples of fiber couplers and mode expanders, which, in some embodiments, the ideas in this disclosure could be combined with and/or work as a substitute for.

BACKGROUND

This application relates to optical waveguides. More specifically, and without limitation, this application relates to devices and methods for coupling a silicon waveguide to an optical fiber.

Photonic devices, including optical waveguides, are being integrated on semiconductor chips. Photonic devices integrated on semiconductor chips are often designed for use in fiber-optic communication systems. Improved devices and methods for coupling a silicon waveguide to an optical fiber are desirable.

BRIEF SUMMARY

Embodiments of the invention include multistage spot size converters for coupling a smaller optical mode, such as a fundamental mode in a semiconductor waveguide, to a larger optical mode, such as a fundamental mode in an optical fiber.

According to some embodiments of the present invention, a device is provided for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber. The device includes a waveguide comprising a waveguide taper region, which comprises a shoulder portion and a ridge portion above the shoulder portion. The ridge portion has a width that tapers to meet a width of the shoulder portion. The waveguide taper region comprises a first material. The device also has a mode converter coupled to the waveguide. The mode converter includes a plurality of stages, and each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region. The mode converter is made of a second material different from the first material.

In some embodiments of the above device, the mode converter comprises a first stage and a second stage of the plurality of stages. The second stage overlies the first stage. The first stage includes a first portion that does not taper and a second portion that tapers. The second stage extends over the first portion and the second portion of the first stage.

In some embodiments, the first portion of the first stage of the mode converter and the shoulder portion of the waveguide have a common width.

In some embodiments, the waveguide also includes a waveguide extension region coupled between the waveguide taper region and the mode converter. The waveguide extension region is made of the first material. The waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region. The waveguide extension region and the shoulder portion of the waveguide taper region have a common width. In some embodiments, the waveguide extension region has a common thickness as the waveguide taper region and a first stage of the mode converter. In some embodiments, the waveguide extension region has a common width as an output end of the waveguide taper region and an input end of a first stage of the mode converter.

In some embodiments, the number of stages in the mode converter is greater than three and less than eleven. In some embodiments, the number of stages in the mode converter is seven.

In some embodiments, the ridge portion tapers from a narrower width of 0.75-2.75 μm near an input end to a wider width of 2.0-4.0 μm near an output end. In some embodiments, the waveguide taper region, including the ridge portion and the shoulder portion, has a height of 0.75-2.25 μm.

In some embodiments, at an output end of the mode converter, the mode converter has a rectangular (e.g., square) cross-section, and the plurality of stages has a common maximum width.

In some embodiments, the mode converter comprises a first stage and a second stage of the number of stages. The first stage has a first maximum width, and the second stage has a second maximum width. The first maximum width of the first stage is greater than the second maximum width of the second stage.

In some embodiments, at a cross section of the mode converter at an output end of the device, each stage of the plurality of stages has a different width. A first stage of the plurality of stages, which is closer to the shoulder portion, has a wider width than a second stage of the plurality of stages, which is farther from the shoulder portion than the first stage.

In some embodiments, the first stage of the mode converter has a first length, the second stage of the mode converter has a second length, and the first length is equal to the second length. In some embodiments, at a cross section of the mode converter at an output end of the device, each stage of the plurality of stages has a common width, and the mode converter has a rectangular cross-section.

In some embodiments, the first material is crystalline silicon and the second material is non-crystalline silicon. The non-crystalline silicon can include amorphous silicon and polycrystalline silicon.

In some embodiments, the waveguide taper region and the mode converter are integrated on a substrate that includes a buried-oxide layer overlying a handle layer.

In some embodiments, the waveguide taper region comprises crystalline silicon in a device layer disposed on a buried-oxide layer of an SOI (silicon-on-insulator) substrate. The mode converter comprises non-crystalline silicon formed on the buried-oxide layer of the SOI substrate.

According to some embodiments of the present invention, a method is provided for converting optical mode spot size between a semiconductor waveguide and an optical fiber. The method includes receiving a light beam at a waveguide comprising a waveguide taper region. The waveguide taper region comprises a shoulder portion and a ridge portion above the shoulder portion. The ridge portion has a width that tapers to meet a width of the shoulder portion, and the waveguide taper region comprises a first material. The method includes expanding the light beam in the waveguide taper region. The method further includes coupling the light beam from the waveguide taper region to a mode converter. The mode converter comprises a plurality of stages. Each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region. The mode converter is made of a second material different from the first material. The method includes expanding the light beam through each of the plurality of stages in the mode converter. The method further includes coupling the light beam to the optical fiber.

In some embodiments of the above the method, the method also includes transmitting the light beam through a first stage of the mode converter to a second stage. The first stage comprises a first portion that does not taper and a second portion that tapers. Further, the second stage extends over the first portion and the second portion of the first stage.

In some embodiments of the method, the method also includes transmitting the light beam through a waveguide extension region coupled between the waveguide taper region and the mode converter. The waveguide extension region is made of the first material. The waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region. The waveguide extension region and the shoulder portion of the waveguide taper region have a common width.

According to some embodiments of the present invention, a device is provided for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber. The device includes a waveguide and a mode converter. The waveguide includes a waveguide taper region and a waveguide extension region coupled to the waveguide taper region. The waveguide taper region includes a shoulder portion and a ridge portion above the shoulder portion. The ridge portion has a width that tapers to meet a width of the shoulder portion. The waveguide taper region is made of a first material, and the waveguide extension region is made of the first material. The waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region. The waveguide extension region and the shoulder portion of the waveguide taper region have a common width. The mode converter is coupled to the waveguide extension region. The mode converter includes a plurality of stages, each of the plurality of stages tapering in a direction similar to a direction of taper of the waveguide taper region. The mode converter includes a first stage and a second stage of the plurality of stages. The first stage comprises a first portion that does not taper and a second portion that tapers, and the second stage extends over the first portion and the second portion of the first stage. The mode converter is made of a second material different from the first material.

In some embodiments of the above device, the ridge portion tapers from a narrower width of 0.75-2.75 μm near an input end to a wider width of 2.0-4.5 μm near an output end. In some embodiments, the waveguide extension region has a common thickness as the waveguide taper region and the first stage of the mode converter.

Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating various embodiments, are intended for purposes of illustration only and are not intended to necessarily limit the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view diagram illustrating a multistage spot size converter according to some embodiments of the present invention;

FIG. 2 is a top view diagram illustrating the multistage spot size converter of FIG. 1 according to some embodiments of the present invention;

FIG. 3 is a cross-sectional view diagram illustrating an input side of the waveguide taper region of the multistage spot size converter of FIGS. 1 and 2 according to some embodiments of the present invention;

FIG. 4 is a cross-sectional view diagram illustrating an output side of the waveguide taper region 110 of the multistage spot size converter 100 of FIG. 1;

FIG. 5 is a cross-sectional view diagram illustrating an input side of the waveguide extension region 120 of the multistage spot size converter 100 of FIGS. 1 and 2;

FIG. 6 is a cross-sectional view diagram illustrating an output side of the waveguide extension region 120;

FIGS. 7A-7G are cross-sectional view diagrams illustrating cross sections of the mode converter 130 of the multistage spot size converter 100 of FIGS. 1 and 2;

FIG. 8 is a longitudinal cross-sectional view diagram illustrating the multistage spot size converter of FIG. 1 according to some embodiments of the present invention;

FIG. 9 is a graph illustrating simulated optical signal strength along the longitudinal cross section of the multistage spot size converter of FIG. 1 according to some embodiments of the present invention;

FIG. 10 is a cross-sectional view diagram illustrating the multistage spot size converter coupled to an optical fiber according to some embodiments of the present invention;

FIG. 11 is a perspective view diagram illustrating a multistage spot size converter according to an alternative embodiment of the present invention;

FIG. 12 is a top view diagram illustrating the multistage spot size converter of FIG. 11 according to some embodiments of the present invention;

FIG. 13 is an input side cross-sectional view diagram illustrating the multistage spot size converter of FIG. 7 according to some embodiments of the present invention;

FIG. 14 is an output side cross-sectional view diagram illustrating the multistage spot size converter of FIG. 7 according to some embodiments of the present invention; and

FIG. 15 shows a flowchart of an embodiment of a method for converting optical mode spot size between a semiconductor waveguide and an optical fiber according to some embodiments of the present invention.

In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

DESCRIPTION

The ensuing description provides preferred exemplary embodiment(s), and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment. It is understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims.

This application discloses embodiments of a mode expander a spot size converter (SSC) for coupling a smaller optical mode, such as a fundamental mode in a semiconductor waveguide, to a larger optical mode, such as a fundamental mode in an optical fiber (e.g., one or more SSCs to couple one or more waveguides of an optical transceiver to one or more optical fibers in a quad small form-factor pluggable (QSFP) module).

FIG. 1 is a perspective view diagram illustrating a multistage spot size converter, and FIG. 2 is a top view diagram illustrating the multistage spot size converter of FIG. 1 according to some embodiments of the present invention. In FIGS. 1 and 2, similar components and/or features may have the same reference label. Further, FIG. 2 includes cut lines 140-148 indicating locations in the device where cross-sectional illustration will be described.

As shown in FIGS. 1 and 2, the spot size converter 100 includes a waveguide taper region 110, an extension region 120, and a mode converter 130. In some embodiments, the extension region can be omitted. The spot size converter 100 is configured to optically couple a semiconductor waveguide, to the right of the waveguide taper region 110, with an optical fiber, to the left of the mode converter 130. Both the semiconductor waveguide and optical fiber are not shown in FIGS. 1 and 2, but are illustrated in FIG. 10. In this embodiment, the waveguide taper region 110, the extension region 120, and the mode converter 130 can be integrated on a SiO₂ layer 150 on a handle substrate 160 in a silicon-on-insulator (SO1) wafer.

The waveguide taper region 110 includes a shoulder portion 112 and a ridge portion 114. The waveguide taper region 110 has an input end 116 and an output end 118. In some embodiments, the input end 116 is coupled to a waveguide (not shown). The waveguide can have a rectangular cross section. In some embodiments, the waveguide can have a cross section that is not rectangular (e.g., trapezoidal or inverted ‘T’ shape with a crystalline-silicon ridge on top of a crystalline-silicon shoulder). The output end 118 is coupled with the extension region 120. Alternatively, the output end 118 can be coupled directly to the mode converter 130.

In FIG. 1, the shoulder portion 112 has a width w-s. The ridge portion 114 tapers from the input end 116 to the output end 118. The ridge has an initial width, w-r, at the input end 116. The width of the ridge portion 114 expands from w-r at the input end 116 to w-s at the output end 118. The waveguide taper region 110 helps relax alignment tolerances for coupling to the extension or mode converter. The ridge portion 114 tapers from the input end 116 to the output end 118 so that an optical beam expands as it is transmitted from the input end 116 to the output end 118. For example, a beam can be transmitted from an external waveguide (not shown), through the input end 116 through the output end 118 of the waveguide taper region 110, and to an optical fiber having a larger core than the waveguide. As the beam propagates through the waveguide taper region 110, the beam expands adiabatically. A direction of beam propagation 124 is shown as an arrow going from the input end 116 to the output end 118.

FIG. 3 is a cross-sectional view diagram illustrating an input side of the waveguide taper region of the multistage spot size converter of FIGS. 1 and 2 according to some embodiments of the present invention. FIG. 3 depicts a cross section of the input end 116 of the waveguide taper region 110 of the spot size converter 100 in FIGS. 1 and 2, along the cut line 140 in FIG. 2. The waveguide taper region 110 at the input end comprises the shoulder portion 112 and the ridge portion 114. The shoulder portion 112 has a thickness, t-s, and a width, w-s. In some embodiments, the width, w-s, of the shoulder portion 112 can range between 0.75 and 4.5 μm (e.g., 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.5, 4, or 4.5 μm). The thickness (sometimes referred to as height) of the shoulder t-s can range between 0.2 and 2 μm (e.g., 0.2, 0.5, 1.0, 1.5, or 2 μm). The combined thickness t1 of the shoulder portion 112 and the ridge portion 114 can range between 0.75 and 2.25 μm (e.g., 1.0, 1.5, 1.75, 2, or 2.25 μm). The shoulder portion 112 is made of crystalline silicon (c-Si). In some embodiments, the shoulder portion 112 can be simply an extension of an external waveguide (not shown) fabricated on a substrate (e.g., a substrate comprising silicon on insulator and/or SiO2). In some embodiments, the waveguide taper region 110 can further comprise a substrate and/or cladding material.

FIG. 4 is a cross-sectional view diagram illustrating an output side of the waveguide taper region 110 of the multistage spot size converter 100 of FIG. 1. FIG. 4 depicts a cross section of the output end 118 of the waveguide taper region 110 of the spot size converter 100 in FIGS. 1 and 2, along the cut line 141 in FIG. 2. The waveguide taper region 110 at the output end comprises the shoulder portion 112 and the ridge portion 114. It can be seen that, in this example, the shoulder portion 112 at the output end has the same width, w-s, as the input end. On the other hand, the ridge portion 114 has tapered and expanded to a width of w-s.

FIG. 5 is a cross-sectional view diagram illustrating an input side of the waveguide extension region 120 of the multistage spot size converter 100 of FIGS. 1 and 2, and FIG. 6 is a cross-sectional view diagram illustrating an output side of the waveguide extension region 120. In some embodiments, the waveguide extension region 120 has a constant width, w-s, matching the width at the output end of the waveguide taper region 110. Therefore, the width, w-s, of the waveguide extension region 120 can range between 0.75 and 4.5 μm (e.g., 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.5, 4, or 4.5 μm). The thickness (sometimes referred to as height) of waveguide extension region 120 can be the same as the combined thickness, t1, of the shoulder portion 112 and the ridge portion 114, and can range between 0.75 and 2.25 μm (e.g., 1.0, 1.5, 1.75, 2, or 2.25 μm). The waveguide extension region 120 can be made of crystalline silicon (c-Si). In some embodiments, the waveguide extension region 120 can be fabricated in the crystalline silicon device layer of an SOI substrate (e.g., a substrate comprising silicon on insulator and/or SiO2). In some embodiments, the waveguide extension region 120 can further comprise a substrate and/or cladding material.

Referring back to FIGS. 1 and 2, the spot size converter 100 comprises the mode converter 130. The mode converter 130 comprises a plurality of stages. In this embodiment, the spot size converter 100 includes seven stages, 131-137. In FIG. 2, an input end is shown at cut line 142, and an output end is shown at cut line 148. An optical beam is expanded going from the input end at 142 to the output end at 148. Even though FIGS. 1 and 2 show the mode converter 130 having seven stages, other embodiments of mode converters can have fewer or more stages.

Each stage has a beginning width, w-b, which is the most narrow part of the stage. In some embodiments, one or more stages have different beginning widths, w-b.

Each stage has a thickness, e.g., measured in a direction normal to a surface of a substrate that the spot size converter 100 is on. The first stage has a first thickness, t1; the second stage has a second thickness, t2; and so on to the n-th stage. There is a total thickness, t(total), which is equal to the sum of the thickness of each stage. Each stage has a length. The length of the first stage is L1, the length of the second stage is L2, and so on to the n-th stage. In the embodiment shown, n=7. The first stage 131 is divided into a first portion 131-1 with length L1′ and a second portion 131-2 with a length of L1“. The first stage 131-1 does not taper along the length L1”. The second stage 131-2 tapers along the length L1′. In some embodiments, L2=L1′+L1″. In some embodiments, splitting the first stage into two portions helps with symmetry between x (width) and y (thickness) mode confinement. In some embodiments, the length of the second stage can be shorter than the length of the first stage, i.e., L2<L1′+L1″.

A table of dimensions of the components in spot size converter 100 depicted in FIGS. 1-6 is shown below. The ranges and values below are meant to be exemplary, and not meant to limit the scope of the invention. In some embodiments, ranges of dimensions below are used to adiabatically expand an optical mode from a silicon waveguide to an optical fiber. In some embodiments, ranges below are used to adiabatically expand an optical mode from a silicon waveguide to an optical fiber in a compact distance to save room on a chip.

Dimension Example Ranges (μm) Example Values (μm) L1′ 200-900 637, 750, 792 L1″ 100-500 208, 250, 363 L2  200-1200 900, 1000 L3  200-1080 917, 925, 933 L4 100-975 782, 800, 833 L5 100-880 708, 725, 728 L6 100-790 484, 500, 521 L7  50-710 300, 333, 350 t1 0.75-2.25 1.0, 1.5, 1.75 t2 0.3-1.5 0.6, 0.7, 0.8 t3 0.5-2.2 1.0, 1.1, 1.2 t4 0.9-3.6 1.5, 1.6, 1.8 t5 1.1-3.8 2.0, 2.1, 2.2 t6 1.3-4.3 2.3, 2.5, 2.6 t7 1.5-4.6 2.8, 2.9, 3.0 t(total)  8-17 12, 12.5, 12.9 w-r 0.75-3.5, 1.0-2.0 1.3, 1.5, 1.75 w-s 0.75-4.5  2.5, 3, 3.5 w1  8-15 11. 5, 11.7, 12, 12.7 w7  9-20 12, 14.1, 14.3, 15 w-delta 0.1-0.5 0.2, 0.3, 0.4 w-b 0.0-1.0 0.3, 0.4, 0.9

In mode converter 130, stage 1 thickness, t1, is equal to a thickness of the waveguide taper region 110 in the waveguide (e.g., thickness of the shoulder portion plus thickness of the ridge portion). From stage 2, thickneseses of subsequent stages (higher-numbered stages) are thicker than preceding stages (lower-numbered stages). In some embodiments, L1′ is more than twice the length of L1″ and/or L2 is 1000 μm or less. In some embodiments, the length of stages are 50%-90% of the stage below (e.g., directly below).

In some embodiments, a number of stages other than seven for the mode converter are used. Though performance can be increased with a higher number of stages, complexity and difficulty in fabricating also increase. In an embodiment, a device with seven stages was found to be better than another device with six stages.

In some embodiments, the beginning width (“tips”) of tapers in the mode converter, (w-b), may be 300 nm or narrower. In some embodiments, 300 nm is a dimension of a photolithography lens used to fabricate the device, but the tips can be much narrower (e.g., 10 nm). In some embodiments, the tips can also be thicker. Also, edges of tapers may not be as sharp as shown, but instead are somewhat rounded, resulting in a design closer to a vertical taper (e.g., instead of vertically stepped profile) than what is depicted in the figures.

FIGS. 7A-7G are cross-sectional view diagrams illustrating cross sections of the mode converter 130 of the multistage spot size converter 100 of FIGS. 1 and 2. More particularly, FIG. 7A depicts a cross section of the mode converter 130 at the cut line 142 shown in FIG. 2, where the first stage 131 has a width of w-s matching the width of the waveguide extension region 120 or the width of the waveguide taper region 110. At cut line 142, the tip of the second stage 132 has a width of w-b.

FIG. 7B depicts a cross section of the mode converter 130 at the cut line 143 in FIG. 2, showing a tip of the third stage 133 having a width w-b. Similarly, FIGS. 7C-7F depict cross sections of the mode converter 130 at the cut lines 143-147 in FIG. 2, showing the tips of the four through seventh stages 134-137, respectively, having a width w-b. FIG. 7G depicts a cross section of the output end of the mode converter 130 at the cut line 148 in FIG. 2. FIG. 7G illustrates that, at an output end of the mode converter 130, the plurality of stages has a common maximum width w7, and the mode converter can have a square or rectangular cross-section.

The multiple stages in mode converter 130 can be of non-crystalline silicon. In this disclosure, examples of non-crystalline silicon include amorphous silicon (a-Si) and polycrystalline silicon (poly-Si).

In some embodiments, the spot size converter 100, including the waveguide taper region 110, the extension region 120, and the mode converter 130, can be formed on a single substrate. For example, the waveguide taper region 110, the extension region 120, and the mode converter 130 can be integrated on a SiO₂ layer 150 on a handle substrate 160 in a silicon-on-insulator (SO1) wafer. In some embodiments, the waveguide taper region 110 and the extension region 120 can be formed in a crystalline silicon (c-Si) layer of a silicon-on-insulator (SO1) substrate, and the mode converter 130 can be made of non-crystalline silicon. In some embodiments, the ridge portion 114 of the waveguide taper region 110 can also be made of non-crystalline silicon. In some embodiments, the shoulder portion 112 can also be made of non-crystalline silicon. In some embodiments, the extension region 120 can also be made of non-crystalline silicon. In some embodiments, stage 1 of the mode converter can be made of crystalline silicon, and the other stages above stage 1 can be made of non-crystalline silicon. More details of the method for making the various structures are described in U.S. patent application Ser. No. 14/722,983, filed on May 27, 2015, incorporated by reference in its entirety for all purposes.

In crystalline silicon, a lattice structure is well defined. In non-crystalline silicon, a lattice structure is not well defined. Examples of non-crystalline silicon include amorphous silicon (a-Si) and polycrystalline silicon (poly-Si). In polycrystalline silicon, the lattice structure is not well defined, and a polycrystalline-silicon structure comprises multiple crystal lattices. In some embodiments, though non-crystalline silicon may have more loss than crystalline silicon, non-crystalline silicon is used for manufacturing reasons (e.g., for manufacturing tolerances and/or for expanding a beam larger than a crystalline-silicon layer). Another advantage of non-crystalline silicon, in some embodiments, is that non-crystalline has a stable and predictable index of refraction that is similar to crystalline silicon (e.g., the crystalline silicon has a first index of refraction; the non-crystalline silicon has a second index of refraction; and the first index of refraction minus the second index of refraction is less than 0.05, 0.1, 0.2, or 0.3).

FIG. 8 is a longitudinal cross-sectional view diagram illustrating the multistage spot size converter of FIG. 1 according to some embodiments of the present invention. FIG. 8 shows a cross section of the spot size converter 100 through the center of the waveguide taper region 110, the extension region 120, and the mode converter 130, which are integrated on a SiO₂ layer 150 on a handle substrate 160 in a silicon-on-insulator (SO1) wafer. The spot size converter 100 is configured to optically couple a semiconductor waveguide, to the right of the waveguide taper region 110, with an optical fiber, to the left of the mode converter 130. Both the semiconductor waveguide and optical fiber are not shown in FIG. 8, but are illustrated in FIG. 10. The mode converter 130 includes multiple stages 131-137, each having a thickness t1-t7 and a length L1-L7, respectively.

FIG. 9 is a graph illustrating the simulated optical signal strength along the longitudinal cross section of the multistage spot size converter of FIG. 1 according to some embodiments of the present invention. In FIG. 9, a simulated profile of a beam is shown, with the density of the black dots indicating optical signal strength. It can be seen that the small spot size of the beam with high intensity, at the input end on the left hand side, is expanded as it is transmitted along the longitudinal direction through the waveguide taper region 110, the waveguide extension region 120, and the mode converter 130. As the spot size of the beam is expanded, the signal strength density is reduced.

FIG. 10 is a cross-sectional view diagram illustrating the multistage spot size converter coupling waveguide to an optical fiber according to some embodiments of the present invention. FIG. 10 shows a side view of an embodiment of a spot size converter (SSC) 100 optically coupled (e.g., butt coupled) to an optical fiber 200. The spot size converter 100 includes a waveguide taper region 110, an extension region 120, and a mode converter 130, which are integrated on a SiO₂ layer 150 on a handle substrate 160 in a silicon-on-insulator (SO1) wafer. The spot size converter (SSC) 100 can be fabricated on the same SOI wafer as a waveguide 10, which can be extended to form the waveguide taper region 110. The optical fiber 200 can include cladding regions 208 surrounding a core region 204. The SSC and optical fiber are well matched so that a lens t is not used to couple light from the SSC to the optical fiber.

FIG. 11 is a perspective view diagram illustrating a multistage spot size converter according to an alternative embodiment of the present invention, and FIG. 12 is a top view diagram illustrating the multistage spot size converter of FIG. 11. In FIGS. 11 and 12, similar components and/or features may have the same reference label. The description below may refer to reference labels in one or both of FIGS. 11 and 12.

As shown in FIGS. 11 and 12, the spot size converter 1100 includes a waveguide taper region 1110 and a mode converter 1130. In some embodiments, an extension region can be disposed between the waveguide taper region 1110 and the mode converter 1130, similar to the extension region 120 depicted in FIGS. 1 and 2. The spot size converter 1100 is configured to optically couple a semiconductor waveguide, to the right of the waveguide taper region 1110, with an optical fiber, to the left of the mode converter 1130. Both the semiconductor waveguide and optical fiber are not shown in FIGS. 11 and 12.

The waveguide taper region 1110 is similar to waveguide taper region 1110 depicted in FIGS. 1 and 2, and most of the descriptions of the waveguide taper region 110 are also applicable to the waveguide taper region 1110. For example, the waveguide taper region 1110 includes a shoulder portion 1112 and a ridge portion 1114. The waveguide taper region 1110 has an input end 1116 and an output end 1118. In some embodiments, the input end 1116 is coupled to a waveguide (not shown). The waveguide can have a rectangular cross section. In some embodiments, the waveguide can have a cross section that is not rectangular (e.g., trapezoidal or inverted ‘T’ shape with a crystalline-silicon ridge on top of a crystalline-silicon shoulder). The output end 1118 of the waveguide taper region 1110 is coupled to the mode converter 1130. Alternatively, the output end 1118 can be coupled with a waveguide extension region, which, in turn, can be coupled to the mode converter 130.

In FIG. 12, the shoulder portion 1112 has a width w-s. The ridge portion 1114 tapers from the input end 1116 to the output end 1118. The ridge has an initial width, w-r, at the input end 1116. The width of the ridge portion 1114 expands from w-r at the input end 1116 to w-s at the output end 1118. The waveguide taper region 1110 helps relax alignment tolerances for coupling to the extension or mode converter. The ridge portion 1114 tapers from the input end 1116 to the output end 1118 so that an optical beam expands as it is transmitted from the input end 1116 to the output end 1118. For example, a beam can be transmitted from an external waveguide (not shown), through the input end 1116 through the output end 1118 of the waveguide taper region 1110, and to an optical fiber having a core larger than the waveguide 10 and larger than the extension region 120. As the beam propagates through the waveguide taper region 1110, the beam expands adiabatically. A direction of beam propagation 124 is shown as an arrow going from the input end 1116 to the output end 1118.

Referring back to FIGS. 11 and 12, the spot size converter 1100 also includes the mode converter 1130. The mode converter 1130 comprises a plurality of stages. In this embodiment, the mode converter 1130 includes seven stages, 1131-1137. An optical beam is expanded going from an input end to an output end. Even though FIGS. 11 and 12 show the mode converter 130 having seven stages, other embodiments of mode converters can have fewer or more stages.

Each stage has a beginning width, w-b, which is the most narrow part of the stage. In some embodiments one or more stages have different beginning widths, w-b.

The mode converter 1130 is similar to the mode converter 130 depicted in FIGS. 1 and 2, and most of the descriptions of the mode converter 130 are also applicable to the mode converter 1130. One notable difference between the mode converter 1130 and the mode converter 130 is that, at the output end, each stage in the mode converter 130 has the same width, resulting in a square or rectangular cross section. On the other hand, each stage in the mode converter 1130 in FIGS. 11 and 12 has a different width at the output end. As shown in FIG. 12, at the output end, the first stage 1131 has a width w1, the second stage 1132 has a width w2, . . . , and the seventh stage 1137 has a width w7. The width of the stage at a higher location has a narrower width than the one stage below. The difference between successive stages, w-delta, can be the same in some embodiments. In other embodiments, w-delta can be different between different pairs of adjacent stages.

In some embodiments, at a cross section of the mode converter at an output end of the device, each stage of the plurality of stages has a different width. A first stage of the plurality of stages, which is closer to the shoulder portion, has a wider width than a second stage of the plurality of stages, which is farther from the shoulder portion than the first stage. The shape of the output end of the mode converter 1130 at the input end and the output end is further illustrated in FIGS. 13 and 14.

FIG. 13 is a cross-sectional view diagram illustrating an input side of the waveguide taper region of the multistage spot size converter 1130 of FIGS. 11 and 12 according to some embodiments of the present invention. FIG. 13 depicts a cross section of the input end 1116 of the waveguide taper region 1110 of the spot size converter 1100 in FIGS. 11 and 12. The waveguide taper region 1110 at the input end comprises the shoulder portion 1112 and the ridge portion 1114. The shoulder portion 1112 has a thickness, t-s, and a width, w-s. In some embodiments, the width, w-s, of the shoulder portion 112 can range between 0.75 and 4.5 μm (e.g., 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.5, 4, or 4.5 μm). The thickness (sometimes referred to as height) of the shoulder t-s can range between 0.2 and 2 μm (e.g., 0.2, 0.5, 1.0, 1.5, or 2 μm). The combined thickness, t1, of the shoulder portion 1112 and the ridge portion 1114 can range between 0.75 and 2.25 μm (e.g., 1.0, 1.5, 1.75, 2, or 2.25 μm). The shoulder portion 1112 is made of crystalline silicon (c-Si). In some embodiments, the shoulder portion 1112 can be simply an extension of an external waveguide (not shown) fabricated on a substrate (e.g., a substrate comprising silicon on insulator and/or SiO2). In some embodiments, the waveguide taper region 110 can further comprise a substrate and/or cladding material.

FIG. 14 is an output side cross-sectional view diagram illustrating the multistage spot size converter of FIG. 11 according to some embodiments of the present invention. FIG. 14 shows cross sections of seven stages 1131, 1132, . . . , 1137 of the mode converter 1130. It can be seen that each successive stage has a narrower width than the previous stage. The resulting pyramid or triangular shape cross section of the mode converter 1130 can be used to couple to an optical fiber to achieve a different mode conversion.

FIG. 15 is a flowchart summarizing a method 1500 for converting optical mode spot size between a semiconductor waveguide and an optical fiber. The method 1500 includes, at process 1510, receiving a light beam at a waveguide comprising a waveguide taper region, which can have a shoulder portion and a ridge portion above the shoulder portion, the ridge portion having a width that tapers to meet a width of the shoulder portion, the waveguide taper region comprising a first material. At process 1520, the method includes expanding the light beam in the waveguide taper region. The method 1500 further includes coupling the light beam from the waveguide taper region to a mode converter, at 1530. The mode converter comprises a plurality of stages. Each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region. The mode converter is made of a second material different from the first material. At process 1540, the method 1500 includes expanding the light beam through each of the plurality of stages in the mode converter. At process 1550, the method includes coupling the light beam to the optical fiber.

In some embodiments, the method 1500 also includes transmitting the light beam through a first stage of the mode converter to a second stage. The first stage comprises a first portion that does not taper and a second portion that tapers. Further, the second stage extends over the first portion and the second portion of the first stage.

In some embodiments, the method 1500 also includes transmitting the light beam through a waveguide extension region coupled between the waveguide taper region and the mode converter. The waveguide extension region is made of the first material. The waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region. The waveguide extension region and the shoulder portion of the waveguide taper region have a common width.

The specific details of particular embodiments may be combined in any suitable manner without departing from the spirit and scope of embodiments of the invention. However, other embodiments of the invention may be directed to specific embodiments relating to each individual aspect, or specific combinations of these individual aspects.

The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.

A recitation of “a,” “an,” or “the” is intended to mean “one or more” unless specifically indicated to the contrary.

All patents, patent applications, publications, and descriptions mentioned here are incorporated by reference in their entirety for all purposes. None is admitted to be prior art. 

What is claimed is:
 1. A method for manufacturing a device for optical mode spot size conversion, the method comprising: providing a substrate having a device layer disposed on the substrate, wherein the device layer comprises a first material; etching the device layer to form a waveguide; etching the device layer to form a waveguide taper region, wherein: the waveguide taper region comprises a shoulder portion and a ridge portion, the ridge portion oriented along a direction of beam propagation, the ridge portion having a width that tapers to meet a width of the shoulder portion; and forming a mode converter coupled with the waveguide taper region, wherein: the mode converter comprises a plurality of stages; each of the plurality of stages tapers in a direction similar to a direction of taper of the ridge portion of the waveguide taper region; and the mode converter is made of a second material different from the first material.
 2. The method claim 1, wherein the ridge portion of the waveguide taper region is disposed directly above the shoulder portion of the waveguide.
 3. The method claim 1, further comprising: applying a photoresist on the device layer; etching the device layer to form a first recess, the first recess having a shape of a first pattern; removing photoresist from the device layer; filling the first recess with a second material different from the first material; etching the device layer to define the waveguide, and etching the second material to form a first stage of the mode converter, such that the first stage of the mode converter is aligned with the waveguide; covering the substrate with a first cladding; applying photoresist on the first cladding; etching the first cladding to form a second recess, the second recess having a shape of a second pattern; removing photoresist from the first cladding; and filling the second recess with the second material to forms a second stage of the mode converter.
 4. The method claim 3, wherein: the waveguide taper region comprises crystalline silicon in the device layer disposed on a buried-oxide layer of an SOI (silicon-on-insulator) substrate; and the mode converter comprises non-crystalline silicon formed on the buried-oxide layer of the SOI substrate.
 5. The method claim 3, wherein etching the first cladding uses a highly selective etch such that the first cladding is more easily etched than the first material and the second material.
 6. The method claim 3, wherein: the first stage of the mode converter comprises a first portion that does not taper and a second portion that tapers; and the second stage of the mode converter extends over the first portion and the second portion of the first stage.
 7. The method claim 6, wherein the first portion of the first stage of the mode converter and the shoulder portion of the waveguide have a common width.
 8. The method claim 3, further comprising: applying a second cladding on the second stage of the mode converter; etching the second cladding to form a third recess, the third recess having a shape of a third pattern; and filling the third recess with additional second material to form a third stage of the mode converter.
 9. The method claim 8, wherein the number of stages in the mode converter is seven.
 10. The method claim 1, wherein the waveguide further comprises a waveguide extension region coupled between the waveguide taper region and the mode converter, wherein: the waveguide extension region is made from the device layer; the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and the waveguide extension region and the shoulder portion of the waveguide taper region have a common width.
 11. The method claim 10, wherein the waveguide extension region has a common thickness as the waveguide taper region and a first stage of the mode converter.
 12. The method claim 10, wherein the waveguide extension region has a common width as an output end of the waveguide taper region and an input end of a first stage of the mode converter.
 13. A method for manufacturing a device for optical mode spot size conversion, the method comprising: providing a substrate having a device layer disposed on the substrate, the device layer comprising a first material; etching the device layer to form a waveguide and a first stage of a mode converter coupled to the waveguide, wherein the waveguide comprises a waveguide taper region having a shoulder portion and a ridge portion, the ridge portion disposed directly above the shoulder portion and oriented along a direction of beam propagation, the ridge portion having a width that tapers to meet a width of the shoulder portion; depositing a cladding layer over the substrate; forming a first cladding on the device layer; applying photoresist on the first cladding; etching the first cladding to form a first recess, the first recess having a shape of a first pattern; removing photoresist from the first cladding; filling the first recess with a second material to form a second stage of the mode converter.
 14. The method of claim 13, wherein the first material is crystalline silicon and the second material is non-crystalline silicon.
 15. The method of claim 13, wherein the device layer comprises crystalline silicon disposed on a buried-oxide layer of an SOI (silicon-on-insulator) substrate.
 16. The method of claim 13, wherein the waveguide further comprises a waveguide extension region coupled between the waveguide taper region and the mode converter.
 17. The method of claim 16, wherein: the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and the waveguide extension region and the shoulder portion of the waveguide taper region have a common width.
 18. A method for manufacturing a device for optical mode spot size conversion, the method comprising: providing a substrate having a device layer disposed on the substrate, wherein the device layer comprises a first material; etching the device layer to form a waveguide and a waveguide extension region; etching the device layer to form a waveguide taper region, the waveguide taper region coupled to the waveguide extension region, wherein: the waveguide taper region comprises a shoulder portion and a ridge portion, the ridge portion having a width that tapers to meet a width of the shoulder portion; and the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and the waveguide extension region and the shoulder portion of the waveguide taper region have a common width; and forming a mode converter coupled to the waveguide extension region, wherein: the mode converter comprises a plurality of stages; each of the plurality of stages tapers in a direction similar to a direction of taper of the ridge portion of the waveguide taper region; the mode converter comprises a first stage and a second stage of the plurality of stages, the first stage comprising a first portion that does not taper and a second portion that tapers; and the second stage extends over the first portion and the second portion of the first stage; and the mode converter is made of a second material different from the first material.
 19. The method of claim 18, wherein the ridge portion of the waveguide taper region tapers from a narrower width of 0.75-2.75 μm near an input end to a wider width of 2.0-4.5 μm near an output end.
 20. The method of claim 18, wherein the waveguide extension region has a common thickness as the waveguide taper region and the first stage of the mode converter. 